An increased concern about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon have led governmental authorities to place limits on allowable emissions of one or more of such pollutants. Clearly, the trend is in the direction of increasingly stringent restrictions.
NOx, or oxides of nitrogen, in flue gas streams exiting from FCC regenerators is a pervasive problem. FCCUs process hydrocarbon feeds containing nitrogen compounds, a portion of which is contained in the coke on the catalyst as it enters the regenerator. Some of this coke nitrogen is eventually converted into NOx emissions, either in the FCC regenerator or in a downstream CO boiler. Thus all FCCUs processing nitrogen-containing feeds can have a NOx emissions problem due to catalyst regeneration.
In a FCC process, catalyst particles or inventory are repeatedly circulated between a catalytic cracking zone and a catalyst regeneration zone. During regeneration, coke from the cracking reaction deposits on the catalyst particles and is removed at elevated temperatures by oxidation with oxygen containing gases such as air. The removal of coke deposits restores the activity of the catalyst particles to the point where they can be reused in the cracking reaction. The coke removal step is performed over a wide range of oxygen conditions. At the minimum, there is typically at least enough oxygen to convert essentially all of the coke made to CO and H2O. At the maximum, the amount of oxygen available is equal to or greater than the amount necessary to oxidize essentially all of the coke to CO2 and H2O.
In a FCCU operating with sufficient air to convert essentially all of the coke on the catalyst to CO2 and H2O, the gas effluent exiting the regenerator will contain “excess oxygen” (typically 0.5 to 4% of total off gas). This combustion mode of operation is usually called “complete burn” or “full burn”. When the FCCU regenerator is operating in full burn mode, the conditions in the regenerator are for the most part oxidizing. That is, there is at least enough oxygen to convert (burn) all reduced gas phase species (e.g., CO, ammonia, HCN) regardless of whether this actually happens during the residence time of these species in the regenerator. Under these conditions, essentially all of the nitrogen deposited with coke on the catalyst during the cracking process in the FCCU riser is eventually converted to molecular nitrogen or NOx and exits the regenerator as such with the off gas. The amount of coke nitrogen converted to NOx as opposed to molecular nitrogen depends on the design, conditions and operation of the FCCU, and especially of the regenerator, but typically the majority of coke nitrogen exits the regenerator as molecular nitrogen.
On the other hand, when the amount of air added to the FCCU regenerator is insufficient to fully oxidize the coke on the cracking catalyst to CO2 and H2O, some of the coke remains on the catalyst, while a significant portion of the coke carbon burned is oxidized only to CO. In FCCUs operating in this fashion, oxygen may or may not be present in the regenerator off gas. However, should any oxygen be present in the regenerator off gas, it is typically not enough to convert all of the CO in the gas stream to CO2 according to the chemical stoichiometry ofCO+½O2→CO2 This mode of operation is usually called “partial burn.” When a FCCU regenerator is operating in partial burn mode, the CO produced, a known pollutant, cannot be discharged untreated to the atmosphere. To remove the CO from the regenerator off gas and realize the benefits of recovering the heat associated with burning it, refiners typically burn the CO in the regenerator off gas with the assistance of added fuel and air in a burner usually referred to as “the CO boiler”. The heat recovered by burning the CO is used to generate steam.
When the regenerator is operating in partial burn, the conditions in the regenerator, where the oxygen added with air has been depleted and CO concentration has built up, are reducing. That is, there is not enough oxygen to convert/burn all reduced species regardless if some oxygen is actually still present. Under these conditions some of the nitrogen in the coke is converted to so called “reduced nitrogen species”, examples of which are ammonia and HCN. Small amounts of NOx may also be present in the partial burn regenerator off gas. When these reduced nitrogen species are burnt in the CO boiler with the rest of the regenerator off gas, they can be oxidized to NOx, which is then emitted to the atmosphere. This NOx along with any “thermal” NOx formed in the CO boiler burner by oxidizing atmospheric N2 constitute the total NOx emissions of the FCCU operating in a partial or incomplete combustion mode.
FCCU regenerators may also be designed and operated in an “incomplete burn” mode intermediate between full burn and partial burn modes. An example of such an intermediate regime occurs when enough CO is generated in the FCCU regenerator to require the use of a CO boiler, but because the amounts of air added are large enough to bring the unit close to full burn operation mode, significant amounts of oxygen can be found in the off gas and large sections of the regenerator are actually operating under oxidizing conditions. In such case, while gas phase reduced nitrogen species can still be found in the off gas, significant amounts of NOx may also be present. In most cases a majority of this NOx is not converted in the CO boiler and ends up being emitted to the atmosphere.
Yet another combustion mode of operating a FCCU is nominally in full burn with relatively low amounts of excess oxygen and/or inefficient mixing of air with coked catalyst. In this case, large sections of the regenerator may be under reducing conditions even if the regenerator is nominally oxidizing. Under these conditions, reduced nitrogen species may be found in the regenerator off gas along with NOx.
Where both oxidizing and reducing regions exist simultaneously within the FCCU regeneration zone, heterogeneous modes of combustion may develop. For example, as the operation conditions within a FCCU regenerator approaches the transition point between full and partial burn (or incomplete burn) combustion modes, oxidizing and reducing regions may exist in the regenerator. Heterogeneous combustion modes may also exist in a FCCU regenerator where the coke content of the catalyst particles is not uniform across the radial or axial dimension of the regenerator, or where oxygen, CO, CO2, NOx, SOx and gas phase reduced nitrogen and sulfur species formed during regeneration are not uniformly distributed across the radial or axial dimension of the regenerator. Due to poor mixing, the pollutants formed in each area (e.g., CO, SOx, reduced nitrogen and sulfur species in the reducing areas, and SOx and NOx in the oxidizing areas) may not have sufficient time to react with gases from other areas and produce a flue gas having the expected composition for the nominal mode of operation of the regenerator. As a result, the flue gas will contain both oxidized and reduced species.
Some regenerators are mainly a large vessel containing the fluidized catalyst being regenerated, while others employ advanced designs to improve catalyst regeneration and the mixing efficiency of coked catalyst and air, or to allow the burning of more coke without overheating the regenerator. Additional complexity is introduced by the different air grid designs employed for air distribution, and the various catalyst/flue gas separation system designs used. The design of the regenerator, the mode of operation, the wear and tear of the equipment during operation, the type and location of the air distribution device (air ring), the fluidized catalyst bed (dense phase bed level), and other factors result in commercial regenerators which often have heterogeneous modes of combustion. The heterogeneity may be in the mixing of coked catalyst coming from the stripper and its distribution throughout the vessel, especially in the angular dimension. Alternatively, the heterogeneity may be in the mixing of air with the catalyst. The result can be a heterogeneous coke distribution on the cracking catalyst inventory being regenerated and/or a heterogeneous composition of the gas phase throughout the regenerator. All these heterogeneities may occur simultaneously. In this respect the depth of the catalyst dense phase bed (dimension L) versus the diameter of the regenerator (dimension D) is important in facilitating the evolution of heterogeneities in the FCCU regenerator. For example, a low L/D value can be conducive to creating both coked catalyst and air maldistribution. In extreme cases, when L/D is too low for the air superficial velocity employed, channeling of gases through the bed may occur. Thus, the condition exists where air moves through the catalyst bed as a continuous stream, forming few or no bubbles, and allowing little or no contact with the surrounding catalyst and gases. A large L/D value can also result in increased catalyst traffic through the cyclones, increase back pressure and impact the distribution of both spent catalyst and air throughout the regenerator vessel.
Further, heterogeneous combustion modes may also exist in a FCCU regeneration zone comprised of multistage or multiple regeneration vessels. In this case, the catalyst regeneration zone consists of two or more regenerator vessels, each one optionally operating in a different combustion mode or, alternatively each regenerator vessel may have catalyst and/or gas maldistribution and independently operate in a heterogeneous mode of combustion. Typically, in these types of regeneration zones, the cracking catalyst inventory is circulated from one vessel to the other and then on to the riser and stripper. The flue gas from the first stage may be fed into a second stage, or alternatively, sent downstream.
Afterburn is a clear evidence of a regeneration zone suffering from combustion heterogeneities. FCCU operators typically attempt to minimize afterburn with the addition of CO combustion promoters to the unit to facilitate the oxidation of CO in the dense bed. The CO combustion promoter compositions are typically added to the FCCU unit either as a separate particle additive or as an integral component of the cracking catalyst. Thus, use of a CO combustion promoter to control afterburn is another evidence of a regeneration zone suffering from combustion heterogeneities.
Combustion heterogeneities caused by catalyst or gas maldistribution may also be evident when both O2 and CO are detected in the regenerator effluent. For units nominally in full burn excess O2 in the flue gas of equal or greater than 0.1%, preferably equal or greater than 0.3%, most preferred equal or greater than 0.5%, and CO of at least 100 ppm, preferably at least 50 ppm, and most preferred at least 25 ppm respectively indicate that the regenerator is suffering from maldistribution. For units operating in partial burn or incomplete combustion mode having at least 0.1% CO in the flue gas preferably at least 0.5%, most preferred at least 1%, any amount of excess O2 in the flue gas, preferably, more than 0.05%, most preferred more than 0.1%, is evidence of maldistribution.
Temperature gradients throughout the regenerator vessel or vessels both in the radial and axial dimensions may also be symptoms of heterogeneities caused by catalyst or gas maldistribution. FCCU operators typically measure temperatures in the regenerator at various points in the dense bed, the dilute phase, the cyclones, the plenum (if present) and overhead, and the flue gas pipe. A temperature difference between any temperature measurement point above the dense bed and the average dense bed temperature of 10° F., preferably 20° F., most preferred 30° F. is evidence of maldistribution or heterogeneity existing in the regenerator. In the alternative, a temperature gradient across the axial dimension at any point above the air grid of 10° F., preferably 20° F., most preferably 30° F. is evidence of maldistribution or heterogeneity in the regenerator.
Attempts to control NOx released from a regeneration zone having heterogeneous combustion modes have included adjusting operation conditions of the FCCU during a FCC process. For example, U.S. Pat. Nos. 5,268,089 and 5,382,352 disclose reducing NOx emissions by operating close to the point of transition between full and partial burn mode (i.e., incomplete combustion mode, or full burn with low excess O2). It is believed that this mode of operation allows NOx formed in oxidizing sections of the regenerator, and reduced nitrogen species, e.g. NH3, formed in reducing sections of the regenerator to react with each other effectively reducing NOx emissions. However, even in this mode of operation, NOx emissions persist and any reduced nitrogen species left is converted to NOx in a downstream CO boiler and is emitted as NOx along with any unreacted NOx escaping the regenerator.
Several additive compositions have been proposed for reducing NOx emissions from a FCCU regenerator during a FCC process wherein the regenerator is operating in a specified mode of combustion. For example, U.S. Pat. Nos. 6,129,834; 6,143,167; 6,280,607; 6,379,536; 6,165,933 and 6,358,881 discloses additive compositions which are useful for controlling NOx emissions from a FCCU regenerator operating in a full burn combustion mode. On the other hand, U.S. Pat. No. 6,660,683 discloses additive compositions which are useful for the control of NOx emissions from a FCCU regenerator operating in a partial or incomplete combustion mode. The additive compositions accomplish NOx reduction by converting reduced nitrogen species to molecular nitrogen before they exit the partial or incomplete combustion mode regenerator and are converted to NOx in a downstream CO boiler.
Additive compositions have not been reported for use to control NOx emissions released from a FCCU regeneration zone operating under heterogeneous combustion modes. Clearly, the use of additives under such conditions presents a particularly difficult challenge to the FCCU operator for several reasons. First, since additive compositions are typically circulated throughout the entire FCCU, the additives will be subjected to various combustion conditions. Secondly, each combustion mode requires a different chemistry to reduce NOx emissions released from a FCCU regeneration zone operating under heterogeneous combustion conditions during a FCC process. Thirdly, an additive suited for reducing emissions under one combustion regime may be ineffective under another combustion regime and may even promote an increase of NOx emissions under the other combustion regime.
Consequently, there exists a need in the refining industry for improved processes for reducing NOx emissions released from a FCCU regeneration zone operating under heterogeneous combustion modes during a FCC process, which processes are simple and effective.